Traditional thermal barrier coatings used for rotating airfoils in aircraft engines consist of a strain-tolerant Y2O3-stabilized ZrO2 (YSZ) layer prepared by electron physical vapor deposition (EBPVD) and a metallic bond coating which provides high-temperature oxidation protection. The principal failure mode of the EBPVD-Thermal barrier coatings is progressive fracture along the interface region between the metallic bond coating surface and its thermally grown oxide (TGO) upon oxidation and thermal cycling. The TGO, becomes severely strained with thermal cycling and increased scale thickness due to the large difference in coefficient of thermal expansion (CTE) between the scale and the metal. The metal-TGO interface debonds when the residual and thermal stresses exceed the strength of the interface, particularly in the presence of crack initiation sites such as voids and geometrical roughness at the interface region. Hence, the overall reliability of the EBPVD-thermal barrier coating system is not only dependent on the strain tolerance of the YSZ layer, but is also strongly dictated by the ability of its metallic bond coating to form and retain an adherent TGO upon oxidation and thermal cycling.
Bond coatings commonly used with the EBPVD-YSZ layer are MCrAlY (where M is Ni, Co, or NiCo) prepared by vacuum plasma spray (VPS); and Pt-aluminide prepared by Pt electroplating followed by aluminizing by pack cementation or chemical vapor deposition (CVD). The MCrAlY coating contains small amounts or less than a few percent by weight of what are commonly referred to as reactive elements such as Y, Hf, Zr, etc. which greatly increase TGO adhesion. The presence of Pt in the aluminide coating promotes TGO adhesion. The major role of the reactive elements is to immobilize sulfur impurities and therefore to retard their segregation to the metal-TGO interface. It is believed that the sulfur segregation weakens the degree of chemical bonding (i.e., work of adhesion) at the metal-TGO interface.
In addition, metallic bond coatings contribute significant non-load bearing “dead weight” to thin-walled turbine components. Consideration of a “bond coating-less” approach has become possible because of recent progress in casting of single-crystal Ni-alloys that are doped with a small amount of yttrium or that are desulfurized. These performance characteristics are comparable to and/or superior to those attainable with the best available MCrAlY and Pt-aluminide bond coatings. However, when the yttrium-doped or desulfurized alloys are directly bonded to an EBPVD-YSZ layer without a bond coating, inconsistent performance is generally observed. The inconsistent performance, which contradicts the excellent cyclic oxidation kinetics of these alloys, may be attributed to the inability to control localized surface defects such as Ta-rich areas and impurity contamination at the alloy surface (particularly sulfur) introduced during surface pretreatments prior to the EBPVD deposition.
U.S. Pat. No. 4,880,614 describes the presence of an Al2O3 interlayer (1 μm thick) prepared by CVD on the surface of an MCrAlY bond coating increased the burner-rig life of an EBPVD-TBC by about five-fold. Also, Sun et al. 1993 Oxid. Metals 40:465–481 describe the presence of a CVD Al2O3 layer (4 μm thick) between a plasma sprayed YSZ layer and an NiCoCrAlY bond coating layer which substantially increased the cyclic oxidation life of the YSZ layer. The rate of bond coating oxidation was reduced because of the presence of the artificial Al2O3 layer, and the formation of spinels was not observed at the TGO-YSZ interface.
Both U.S. Pat. No. 4,880,614 and Sun et al. used a CVD process which utilizes AlCl3, CO2, and H2 as precursors at a deposition temperature of about 1000° C. This CVD process was previously developed, and is being widely used for the cutting tool industry. The non-line of sight, atomistic growth technique is attractive for manufacturing, since engineering components with intricate shapes and complex surface features can be readily coated. The CVD process is the only technique currently capable of commercially producing α-Al2O3 in the form of coherent and dense coatings. PVD methods such as sputtering, reactive sputtering, reactive evaporation, ion-assisted deposition, and cathodic arc plasma deposition are generally known to produce metastable or amorphous Al2O3, unless post-deposition annealing above 1000° C. is applied. However, the annealing of metastable and amorphous Al2O3 phases causes extensive microcracking, which is caused by volume reduction during their transformations to the thermodynamically stable α-Al2O3 phase (e.g., about 9% volume reduction for the γ-to-α-Al2O3 transition). Therefore, the annealing approach is undesirable as a method for producing an α-Al2O3 interlayer for oxidation resistance.
The present invention describes a method to increase the oxidation life of thermal barrier coatings while reducing the non-load-bearing weight of turbine components coated with a thermal barrier coating.